In fact...weren't a lot of the male regulars referred to this way? Mr. Sulu, Mr. Chekov...pretty much everybody but the captain and McCoy. They weren't called "Mr." as regularly as Spock and Scotty were, but they were called that a lot, weren't they?
The reason I remember is that I used to wonder why he never referred to females as Miss or whatever. I always kind of assumed it was because TPTB didn't want to fuss about whether they were married or not - Ms. was neither well-known nor accepted in those days.
Hey - maybe this answers the question in that thread about female yeomen? It gives the captain a little variety in his dialog?